1. Field of the Invention
The present invention relates to a head controller, a storage device, and a head controlling method that control using thermal expansion due to a heater, a position of a tip of a head that faces a storage medium and that at least reads signals of the storage medium, and, more particularly to a head controller, a storage device, and a head controlling method that can accurately control a spacing between the head and a disk medium.
2. Description of the Related Art
In a commonly used storage device such as a magnetic disk device and a magneto-optical disk device that stores data in a disk medium, a head that is noncontactably provided with respect to the disk medium reads data from the disk medium and writes data to the disk medium. When reading data from the disk medium, because the head detects signals from a magnet or signals due to reflection of a laser beam, sensitivity to the signals is increasingly enhanced due to the head and the storage medium approaching each other, thus increasing precision of data. Recently, along with an increase in a surface density of the disk medium, levitation that indicates a distance between the head and the disk medium is extremely reduced, for example, to less than or equal to 10 nanometers (nm).
However, along with a reduction in the levitation, a tip of the head touching the disk medium is likely to scratch the head or the surface of the disk medium and interfere with rotations of the disk medium. Thus, accurately positioning the tip of the head and fixedly maintaining a distance (hereinafter, “spacing”) between the tip of the head and a disk medium surface is becoming increasingly significant.
For example, in a technology disclosed in Japanese Patent Application Laid-open No. 2003-272335, a heating coil and a thermal expansion body are included inside the head and thermal expansion of the thermal expansion body is controlled by power supply to the heating coil, thus causing a surface of the head facing the disk medium to protrude in a direction of the disk medium to regulate the spacing. When using a technology such as the technology disclosed in Japanese Patent Application Laid-open No. 2003-272335 to control the spacing by regulating power of a heater such as the heating coil, a prior calibration needs to be carried out for acquiring a correspondence between the heater power and the spacing. In other words, by gradually increasing the heater power, a heater power (hereinafter “touchdown point”) needs to be calculated when the tip of the head and the disk medium surface touch each other (touchdown) and the spacing becomes zero.
When calculating a touchdown point, an increase in a signal level along with the increase in the heater power is monitored and the heater power is acquired at the time when the signal level converges at an upper limit. In other words, due to thermal expansion as a result of the increase in the heater power, the tip of the head approaches near the disk medium surface, thus enhancing the sensitivity to the signals and increasing the signal level. However, the sensitivity is not enhanced after occurrence of the touchdown and the signal level reaches a saturation point. Thus, the heater power at the time when the signal level converges at the saturation point can be detected as the touchdown point.
However, the surface of the disk medium, which is touched by the tip of the head, is not completely smooth and includes minute asperities of a nanometer order. Due to this, the signal level does not converge clearly and detecting the touchdown point becomes difficult. To be specific, as shown in FIG. 5A, if the surface of the disk medium is smooth and ideal, upon increasing the heater power, the signal level increases while maintaining a linearity to a certain extent and upon reaching the upper limit, ceases to increase beyond the upper limit. Thus, during conditions such as the condition shown in FIG. 5A, the touchdown point (indicated by “TDP” in the drawings) can be easily detected.
If the surface of the disk medium includes asperities, as shown in FIG. 5B, the heater power and the signal level maintain the linearity in a range in which the heater power is less and is not affected by the asperities. However, upon the heater power increasing and the tip of the head approaching the disk medium surface, an increase rate of the signal level is desensitized and the signal level reaches the saturation point without a clear point of variation. Due to this, detecting the touchdown point from a change in the signal level becomes difficult and accurate calibration cannot be carried out. If the calibration is not carried out, the spacing cannot be controlled accurately, thus resulting in occurrence of wearing of the disk medium surface and errors while reading data from the disk medium and writing data to the disk medium.